Electronic displays mainly used outdoors, such as mobile devices and digital signage, are provided with a protecting plate for the purpose of protecting the surface of a display.
The surface of such a protecting plate, however, is apt to reflect light, and thus to cause a problem that the display is difficult to view. Therefore, the surface of the protecting plate preferably has a function of preventing image reflection, as well as various functions such as functions of preventing damages and of preventing fouling.
Specific examples of a technique for imparting these functions disclosed include attachment of a protecting plate or an optical filter having functions such as an anti-reflection property, anti-glare property, hardcoating property, antistatic property, anti-fouling property, gas-barrier property, and UV (ultraviolet rays) shielding property, on the display surface (see Patent Document 1, for example). Patent Document 1 discloses an optical filter comprising a near infrared radiation reflective layer including a transparent high-refractive-index thin-film layer and a metal thin-film layer for the purpose of shielding near infrared rays and electromagnetic waves radiated from the screens of displays such as plasma display panels, and an optical filter further comprising a commercially available anti-reflection film stacked thereon and thereby being imparted with anti-reflection and anti-glare properties.
Examples of a common anti-reflection film include AR (anti-reflection) films and LR (low reflection) films which form, on the surface of a substrate, a film having a refractive index different from that of the substrate and reduce reflection owing to the interference effect between light reflected from the substrate surface and light reflected from the surface of the applied film. Further, examples of a common anti-glare film include AG (anti-glare) films which form a film having a fine uneven pattern on the substrate surface, and thus prevent reflection of images owing to the light scattering effect.
Such a light-interference anti-reflection film, however, has a reflectance with a high absolute value and strong wavelength dependence, and thus reflection of the surroundings and coloring due to reflected light occur. Therefore, the film can be further improved in these respects.
In contrast, a moth-eye structure is being focused on as a technique for improving visibility at bright sites different from the above films. The moth-eye structure can provide an ultra-anti-reflection effect without light interference. The moth-eye structure is a structure that a fine uneven pattern, which is finer than the uneven pattern formed on an AG film, is arranged at a pitch of not greater than the visible light wavelength (for example, 400 nm or lower) without any gap on the surface of an article to be anti-reflection-treated, and thereby the refractive index at the boundary between the outside (air) and the article surface is made to pseudo-continuously change. Thereby, most part of light is transmitted regardless of the refractive index interface, and light reflection from the article surface can be almost perfectly prevented (see Patent Document 2, for example).
Patent Document 1: JP 2002-323860 A
Patent Document 2: JP 2004-205990 A
The following will describe one example wherein an anti-reflection film is attached on a protecting plate of a display. FIG. 24 and FIG. 25 each are a schematic cross-sectional view showing one example of a display in which an anti-reflection film is disposed on a protecting plate. FIG. 24 shows the case that an AR film or an LR film is used, whereas FIG. 25 shows the case that a film having a moth-eye structure on its surface (hereinafter, also referred to as a moth-eye film) is used.
As shown in FIG. 24 and FIG. 25, a display generally comprises a display panel 101 for image display and a protecting plate 102 for protecting the display panel 101 with an air layer 103 sandwiched therebetween. Since an observer views an image on the display panel 101 through the protecting plate 102, the protecting plate 102 also serves as an observation port. The protecting plate 102 comprises a substrate 102a and an AR film or LR film 102b, or a moth-eye film 102c, disposed on the substrate 102a. Such a structure reduces reflection of light on the surface of the protecting plate 102, and thus good display can be achieved.
The AR film or LR film 102b shown in FIG. 24 has high surface durability, but the absolute value of its reflectance is high and the light reflected therefrom is to have wavelength dependence. Thus, coloring is observed when the protecting plate 102 is viewed. Further, LR films are designed to primarily reduce the reflectance observed from the front direction. Thus, LR films have particularly high viewing-angle dependence and cause regular reflection of most part of light from oblique directions.
In contrast, the moth-eye film 102c shown in FIG. 25 has extremely low reflectance because most part of light passes through the surface of the protecting plate 102. In addition, the film has very low viewing-angle dependence. Thus, coloring due to reflected light is less observed. The moth-eye film 102c, however, has an uneven structure on its surface, and thus it does not have sufficient surface durability.
FIG. 26 is a graph wherein the reflection spectra of an AR film, LR film, and moth-eye film are compared.
The present invention is devised in consideration of the above current situation, and aims to provide a structure with an observation port which can simultaneously achieve low reflection and suppression of coloring due to reflected light.